The binding of murine T lymphocytes to macrophages will be studied. The kinetics and conditions of binding for normal thymocytes and subpopulations of T cells will be established. Cells from various murine lymphoma lines will be screened for binding and a cell line that resembles a normal thymocyte population will be identified. Preliminary studies indicate that Thy-1 positive lymphoma EL-4 has three properties. A quantitative assay for binding that utilizes radiolabeled lymphocytes will be developed. We will attempt to identify and characterize a macrophage specific binding molecule on the lymphoma cells. Plasma membranes will be purified from the macrophage-binding lymphoma, the membranes will be detergent solubilized and binding by a solubilized receptor from the lymphoma will be tested during inhibition of binding of labeled cells as the assay. If a soluble receptor is identified, we will attempt to purify and characterize it. For this purpose, a site specific antisera, identified by its ability to block the adherence of lymphocytes to macrophages, would be of considerable importance. This would provide unique information about the nature of a molecule involved in specific cell-cell interaction.